1. Field of the Invention
This invention relates generally to emergency lighting devices such as standby automatic lamps, and more particularly to such an emergency lighting device assembled into a common switch cover plate and providing an illumination source and a sensor for determining when AC power has been lost to the electrical switch covered by the plate.
2. Description of Related Art
The following art defines the present state of this field: Nicholl et al., U.S. Pat. No. 4,177,500 describes a power failure light for monitoring a power line and for illumination upon the interruption of power. This invention includes a light bulb, a rechargeable battery, a power supply providing charging current for the battery, a pair of diodes connecting the battery across the power supply, a pair of cascaded transistors connecting the bulb across the battery with base-emitter junctions
Chiu, U.S. Pat. No. 4,461,974 describes a multiple lamp system for use with fiber optic light guide for automatically switching from a main lamp to a standby lamp, upon failure of the former. A solenoid is employed with the inductor in the circuit of the main lamp to overcome the bias of the switch in the standby lamp circuit to keep the former open. When the main lamp fails causing the interruption of current flow, the switch closes thereby permitting energization of the standby lamp. An indicator is provided to show which lamp is functioning.
Jester, U.S. Pat. No. 4,514,789 describes a light switch plate having a rearwardly opening housing for removably holding an AA battery. This invention is detachably mountable over a conventional wall mounted 110 volt AC light switch. An LED mounted on the front of the housing is connected to the battery through an integrated circuit oscillator, which flashes the LED.
McCue et al., U.S. Pat. No. 4,631,649 describes an emergency light fixture that is permanently mountable in a conventional power outlet box having, in fixed relation, a threaded bore for accommodating a screw for a face plate and slots for accommodating an electric plug. The emergency light fixture includes a housing, an electric plug extending from the rear wall of the housing, a battery, a battery charger, and a lamp mounted in the housing and wired to provide illumination during a power outage.
Bavaro et al., U.S. Pat. No. 4,977,351 describes an emergency lighting system which permits at least one gas discharge lamp to be operated from an AC power source when AC current is present and form a battery when AC signal is not present. The circuit also provides the capability for turning the lamp on under other selected emergency conditions such as in response to an intruder detection, or in response to detection of smoke, heat, water, or some other emergency.
Walters et al., U.S. Pat. No. 5,132,596 describes an outdoor lighting control that includes a photosensor responsive to ambient outdoor light and an alternating current relay with a pair of contacts movable between make and break positions. The relay includes a contact actuating arrangement that responds to the photosensor and alternating current bias the contact into a make position and move the contacts electromagnetically into a break position. The contact actuating arrangement is sufficiently stiff and responsive to the alternating current to limit chatter in the contact during passage from make to break to fifteen milliseconds when the photosensor senses a transition between dark and daylight. Preferably, the chatter is limited between 1 and 10 mill-seconds.
Li, U.S. Pat. No. 5,336,977 describes an emergency lighting device that includes a hollow casing, a magnetic field detecting unit, a lamp unit and a driving unit. The casing is positioned adjacent to an electric outlet. The magnetic field detecting unit is disposed inside the casing and detects the absence of a magnetic field from the electric outlet. The lamp unit is mounted operatively on the casing. The driving unit is disposed inside the casing and is connected electrically to the lamp unit and to the detecting unit. The driving unit includes a light-sensitive switch unit and is responsive to the detecting unit so as to activated the lamp unit only when the magnetic field from the electric outlet is absent and there is weak surrounding light.
Blackman, U.S. Pat. No. 5,473,517 describes a housing for an emergency light source, which is electrically connectable to a conventional light switch. The housing replaces a conventional switch plate and has at least one opening for receiving the switch plate and has at least one opening for receiving the switch actuator of the light switch. The replacement housing includes wires for electrical connection to the light switch and is divided into upper and lower housing section, wherein the upper section includes a battery compartment, a printed circuit board compartment, and an opening to receive the switch actuator. The lower housing section includes a compartment for receiving a compact fluorescent lamp, reflectors, and a diffuser cover.
The prior art teaches a variety of safety and emergency lighting devices. Blackman teaches a replacement switchplate with wired connection to a light switch. This switch plate contains a lighting means. Walters teaches a lighting control having photosensor responsive means. Jester teaches a lighted switch plate. Li teaches a magnetic field sensor responsive to an absence of power at a nearby AC outlet, such a sensor being unable to operate in conjunction with a switch since without current flow a magnetic field is not produced. However, the prior art does not teach a switch plate of compact size and proportions having field sensing capability so as to avoid the necessity of hard wiring and which provides an exit marker and illumination. The present invention fulfills these needs and provides further related advantages as described in the following summary.